The capability to form low loss joints between lengths of optical fibers is a basic requisite for the production of fiber communications systems. Such systems, and the underlying theory, are well known and will not be reviewed here. See, for instance, Optical Fiber Telecommunications, S. E. Miller and A. G. Chynoweth, editors, Academic Press, 1979, incorporated herein by reference; in particular, chapter 3, pp. 37-100 therein.
Two types of fiber connections have been developed, one typically being used where relatively frequent connection/disconnection is anticipated, the other is typically being used where this is not the case, and where extremely low loss is important. The latter, usually referred to as a splice, is the type of connection that is the concern of this application.
Currently, there are two general categories of optical fiber splices: fusion splices and butt joint splices. In fusion splices, the ends of two optical fibers are aligned, brought together, and the ends melted by a flame, electric arc, etc., in order to join the ends. In butt joint splices, the ends are aligned, and the fiber ends fixed in the aligned position by means of an appropriate bonding material or mechanical fixture.
Splicing of optical fibers, especially of single mode fibers, requires that the fiber cores be very accurately aligned. Since the core is not always concentric with the outer fiber surface to the necessary accuracy, high precision alignment generally cannot rely on the registry of the fiber surfaces, and techniques for monitoring the relative position of the two fiber cores had to be developed.
The prior art knows a variety of alignment techniques, including alignment using visual observation of the core (e.g., K. Imon and M. Tokuda, Optics Letters, Vol. 8, page 502 (1938); and T. Haibara et al, Optics Letters, Vol. 8, page 235 (1983)) and techniques that comprise sensing of the transmitted or scattered power.
The latter techniques, which can use some form of feedback to control positioning of the cores, include methods that monitor transmitted fundamental mode power, i.e., the power coupled into the receiving fiber and propagating therein in the fundamental fiber mode. This power can be monitored at the far end of the receiving fiber, or it can be monitored locally by stripping some fundamental mode power from the fiber. See Y. Kato et al, Electronics Letters, Vol. 18, page 972 (1982). The former approach is disadvantageous because, inter alia, a signal has to be transmitted back to the splice site. The latter approach, although not subject to that shortcoming, suffers from the inherent drawback that it requires seeking a maximum in the transmitted power, and thus is generally less sensitive than alignment techniques that use radiation scattered from the splice. Furthermore, the fundamental mode technique often necessitates use of radiation of wavelength longer than the operating wavelength of the fiber for alignment. Thus this technique often cannot be used to determine the splice loss at the operating wavelength.
The prior art techniques that use scattered power passively collect scattered radiation by placing a detector or waveguide in the vicinity of the receiving fiber end. See, for instance, A. R. Tynes, Applied Optics, Vol. 9, page 2706 (970). U.S. patent application Ser. No. 367,120, co-assigned with this, discloses a technique for fiber splicing that comprises the use of a waveguide to collect radiation emitted from the receiving fiber near the splice and to transmit the collected radiation to a detector. This and other techniques that passively collect radiation scattered from the splice or near-splice regions of the receiving fiber can typically not easily be made to discriminate between different higher order modes, resulting in a relatively complicated detected power profile as a function of core offset. This in turn complicates the determination of the splice loss, requiring use of a calibration procedure. Furthermore, such techniques typically require apparatus having relatively fragile radiation collecting and/or guiding means, and thus often are not well suited for field application.
In view of the importance of the capability for reliably and conveniently making very low loss splices in optical fiber, especially single mode fiber, and for reliably measuring the splice loss, a splice technique that comprises a sensitive and convenient technique for accurately aligning fiber cores, that uses relatively rugged, field-worthy apparatus and that also has the ability to accurately locally determine the splice loss, is of considerable interest. This application discloses such a splice technique, and apparatus for the practice thereof.